This invention relates to a combination process for upgrading a naphtha fraction. Specifically, the invention relates to a process combining reforming and aromatization over a ZSM-5-type zeolite, to produce a product useful as a high-octane gasoline blending stock or a source from which benzene, toluene and xylene can be recovered.
Also, the production of aromatic hydrocarbons such as benzene, toluene and xylene, and in particular paraxylene, is desired because they are useful as solvents and chemical feedstocks.
Also, in view of the current concern over air pollution and environmental control, processes which will increase the octane number of gasoline while minimizing or eliminating the need for additives are being sought.
One traditional way of increasing the octane of a naphtha fraction has been to subject it to catalytic reforming, usually over a platinum-containing or bimetallic catalyst. In the reformer, naphthenes and paraffins are converted to aromatics, both reactions which substantially increase the octane number of the hydrocarbons involved. Naphthenes are reformed to aromatics with high selectivity. However, the selectivity with which paraffins are converted to aromatics decreases with the number of carbon atoms per paraffin molecule. Only a minor fraction of C.sub.6 paraffins is converted to benzene. Other reactions which occur in the reformer are isomerization and cracking of paraffins. The cracking to C.sub.3 - hydrocarbons represents an irreversible yield loss, and the isomerization of paraffins mainly to singly branched paraffins is a reversible reaction in which a relatively high concentration of low-octane n-paraffins remain in thermal equilibrium with the branched isomers. Thus, inclusion of C.sub.6 paraffinic hydrocarbons in a reformer feed is a less efficient use of the catalyst and reactor facilities than the inclusion of C.sub.6 naphthenes.
Another way of improving the octane number of hydrocarbon fractions is by contacting them with a ZSM-5-type of aluminosilicate zeolite catalyst to produce new aromatic rings from aliphatic compounds. For example, U.S. Pat. No. 3,761,389 teaches aromatization of a hydrocarbon fraction boiling within the range of C.sub.2 to 400.degree. F. with a ZSM-5 type of synthetic aluminosilicate zeolite catalyst, and U.S. Pat. No. 3,756,942 teaches aromatization of a feed consisting essentially of C.sub.5 + paraffins, olefins and/or naphthenes over a ZSM-5-type catalyst to produce a predominantly aromatic liquid and a light hydrocarbon gas. If the aromatization is performed at high temperature (e.g., about 538.degree. C.) and low pressure (e.g., about 1 atmosphere) without added H.sub.2, the light gas includes C.sub.2 -C.sub.4 olefins as well as C.sub.1 -C.sub.4 paraffins.
The art discloses several combinations of reacting a hydrocarbon stream over a reforming catalyst and over a ZSM-5-type catalyst. For example: in U.S. Pat. No. 3,729,409 there is described a process for upgrading a reformate by contacting the reformate and hydrogen with a ZSM-5-type zeolite, to selectively crack the normal paraffins and to form and alkylate aromatic compounds. In U.S. Pat. No. 3,849,290 there is described a process for reforming a naphtha and then removing normal and singly branched hydrocarbons by selective cracking to leave an aromatics-enriched product. In these two processes, some cracking of the alkyl side chain on the aromatic ring occurs, resulting in production of unwanted light ends. In U.S. Pat. No. 3,770,614 there is disclosed a process in which a reformate is fractionated and the light reformate fraction (C.sub.6 to 116.degree. or 127.degree. C.) passed over a ZSM-5-type zeolite to alkylate mono-aromatics. In U.S. Pat. No. 3,950,241 there is disclosed a process for upgrading naphtha by separating it into low- and high-boiling fractions, reforming the low-boiling fraction, and combining the high-boiling naphtha with the reformate for contact with a ZSM-5-type catalyst to crack the paraffins.
Reacting heavy naphthas over ZSM-5-type catalysts in the absence of H.sub.2 and at high temperatures leads to rapid catalyst deactivation, while processing naphthenes over them leads primarily to cracking, which reduces liquid yields.
We have found it advantageous to separate a naphtha into a light fraction and a heavy fraction, reform the heavy fraction, and combine at least a portion of the reformate with the light naphtha fraction to pass over ZSM-5-type catalyst under aromatization conditions, and then to separate a C.sub.5 + product fraction from the aromatized effluent.
In particular, we have provided a process for upgrading a naphtha-boiling-range hydrocarbon to useful products which comprises:
(1) separating the naphtha into a light naphtha fraction containing C.sub.6 aliphatics and lower-boiling hydrocarbons and a heavy naphtha fraction containing methylcyclopentane and higher-boiling hydrocarbons;
(2) reforming the heavy naphtha fraction under reforming conditions to produce a reformate;
(3) passing at least a portion of the reformate together with the light naphtha fraction in contact with a ZSM-5-type zeolite catalyst in at least one of the H-ZSM or Zn-ZSM forms under aromatization conditions including a pressure from 0.5 to 68 atmospheres and a temperature from 316.degree. to 550.degree. C. to produce an aromatics-enriched effluent;
(4) separating the aromatics-enriched effluent into a C.sub.4 - fraction and a C.sub.5 + fraction; and
(5) recovering the C.sub.5 + fraction as the product.